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24                                        CITRIC ACID

citric acid found in the preceding determination, the result is the combined
citric acid (and other acids).

4.  True Citric Acid.—From 15 to 20 c.c. of non-concentrated or about
3 c.c. of concentrated juice are weighed out and neutralised exactly with
approximately 2N-caustic soda solution.    The liquid is diluted to about
50 c.c. and 20 c.c. of about 40% pure calcium chloride solution added ; the
whole is then acidified with a few drops (4-6) of seminonnal hydrochloric
acid, the subsequent procedure being exactly as described for the deter-
mination of citric acid in calcium citrate (.sec Calcium Citrate, 6, second

5.  Alcohol.—Any alcohol present in the juice, is determined by dis-
tillation in the usual way (see Wines, Vol. II),

6.  Sulphuric, Hydrochloric and  Nitric Acids.- -Those adds and
their salts are tested for by the ordinary reagents or by the tests given for
free mineral acids in vinegar (see Vinegar, Vol. 11).    According to Seribani,1
nitric acid is easily detected by adding to the juice (diluted if too highly
coloured) a little hydrochloric acid solution of ferrous chloride free from
ferric salt, boiling and then adding potassium thioeyanate solution ;   if
nitric acid is present in the juice, a red coloration is formed owing to oxida-
tion of the ferrous salt.

7.  Sulphurous Acids, Sulphites..........-The clear, more or less yellow

juices intended for the preparation of syrups and beverages may contain
sulphurous anhydride added as preservative.

(a) Qualitative Test. 50-100 c.c. of the juice are introduced into a
flask which is closed by a cork slit at the bottoxn to allow of the; insertion
of a freshly prepared starch-iodide paper" moistened at the extremity ; if
the flask is heated gently, the paper will assume a blue or brown colour if
sulphurous acid or a sulphite is present.

(&) Quantitative Determination.—Vi\ti\i 50 to 100 c.c. of the juice mixed
with 5 c.c. of 20% phosphoric, acid are distilled in a current of carbon dioxide.
From 25 to 50 c.c. of distillate are collected in 50 c.c. of N/;ro~.iodine solution
contained in a flask with a doubly-bored stopper, through which pass the
condensing tube and a, second tube leading to a U-tube charged with a
definite volume of N/5o-sodium thiosulphate solution (say, 10 c.c.). At
the end of the distillation, the contents of this tube arc washed into the,
iodine solution and the excess of iodine titrated with the sodium thiosulphate.

The number of c.c. of iodine solution used up in oxidising the sulphurous
acid, multiplied by 0-00064, gives the quantity of sulphur dioxide in the

The sulphur dioxide may also be determined gravhnetricaliy on the
distilled product collected in the iodine solution (see Beer, Vol. II).

* *

Besides free citric acid, which represents 88-98% of the total acidity, lemon
juice, etc., contains small proportions of citrates, other free organic acids and

1  Gazzetta cMmica italiana, 1878, VIII, p. 234.

2  The solution for the preparation of this paper is made from o-i gram of potassium
iodide and i gram of starch in 100 c.c. of distilled water.ic salts